The use of oxygen in combustion systems is well known in the art as an effective technique for accelerating the rate of combustion reactions versus that which naturally occurs in air-fuel combustion. Some combustion systems and applications are constrained by such stringent limitations on the time and/or space available for combustion that the use of oxygen-alone (apart from the application of special devices and/or techniques) may not be sufficient to overcome such limitations. For example, in applications in which extremely high speed diffusion (i.e., non-premixed) flames of oxygen and a gaseous fuel are used, the rate of mixing of the two reactant species can be the “rate-limiting” step preventing complete and/or stable combustion. In this case, enhancement of the reactant mixing rate is needed to adequately “feed” the oxygen-fuel reactions.
Another challenging application is the combustion of solid fuel, particularly those solid fuels having extremely low volatile matter, such as anthracite coal and most cokes. Yet another challenging application in the combustion of solid fuel is when the solid fuel must be combusted while undergoing high speed movement and, further, when the same combustion must be completed over a very short reaction distance, such as often occurs with high speed injection of solid fuels into a bed of raw material; for example in an iron-forming cupola or steel-forming blast furnace process.
In some prior art applications, solid fuel is transported in flows with velocities that exceed about 100 m/sec. Such high speed flows generally require exceedingly high transport gas pressures resulting in a very high cost and power requirement, and moreover resulting in very rapid erosion of the walls of the transport passages. Moreover, when the high speed solid fuel is discharged from the transport passage into the combustion space, due to its high momentum, the solid fuel would resist entrainment into even a high speed oxy-gas flame, and would thus fail to adequately heat-up, ignite, and combust as needed within the allowable time and over the allowable distance.
There have been attempts to use cavity-actuated mixing of shear layers to increase combustion rates in high speed flows. In one such prior art system a cavity is placed downstream of the location of initial fuel and oxidizer mixing (the oxidizer being air). In another prior art system, fuel is injected into an air stream upstream from a cavity. Although these systems appear to enhance mixing of oxidizer and fuel, based on Applicants analysis of the likely operating temperatures of such systems, the location of the cavity downstream from initial mixing of the oxidizer and fuel would result in very high temperatures within the conduit in which the mixing occurs—in some cases, temperatures that are substantially higher than the maximum service temperature of most commercially-available steel. Accordingly, the cavity-actuated mixing configurations of the prior art would not be feasible in many applications.
Accordingly, there is a need for improved combustion systems that provide more complete and/or stable combustion in challenging applications, such as those discussed above, while operating within acceptable temperature limits.